l] WHAT IS PLANT GEOGRAPHY ? 5 



world — not to mention their proper delimitation and description. 

 Yet another limitation is the extreme difficulty of collating such 

 complex and often distant ' entities ' as vegetation-types, with all 

 their infinite variation and intricate intergradation. Even so, when 

 no more research than has already been accomplished along these 

 lines is brought together, we have a very impressive volume of 

 material from which it seems permissible to make some useful 

 generalizations, and on which we can build further our edifice of 

 plant geography. 



Plan of the Book 



As plants are our chief concern, we shall, after the present intro- 

 ductory chapter, first describe the main groups into which the 

 myriad forms comprising the plant kingdom (in the wide sense) are 

 classified. For each group we shall give some account of how its 

 members live and reproduce, with mention of their habitats, dis- 

 tributions, and individual importance, and illustrations of examples. 

 Then we shall have at least some conception of what we are dealing 

 with, and, if previously uninitiated, have an opportunity of becoming 

 familiar with our tools. 



Our other main concern being with geographical phenomena and 

 particularly with area, we shall deal next with the physiological 

 attributes and external features that enable particular plants to grow, 

 or prevent them from living, in particular circumstances. In this 

 third chapter we shall also touch on the subject of plant classification 

 by means of the various ' life-forms ' which are brought about largely 

 by the nature of the environment. This is particularly important 

 because the reactions of plants to the environments in which they 

 exist {see pp. 8-9) constitute one of the main ' keys ' to their 

 geographical ranges. In the next chapter we shall consider the means 

 by which plants disperse themselves and migrate — with the kinds 

 of aids they employ and, incidentally, some of the hindrances they 

 meet in attaining their present-day distributions. The following 

 chapter, our fifth, will deal with the early evolutionary development 

 of plants, and the sixth will be concerned particularly with those 

 developments in recent geological ages which have most profoundly 

 influenced plant distributions as we see them nowadays. 



In Chapter VII we shall go on to consider examples of the main 

 types of distribution and consequent areas recognized today, where 

 possible interpreting them in the light of information contained in 



